1. Field of the Invention
The present invention relates to a technique of an electronic source arranged on a negative plate of a FED (Field Emission Display), and particularly relates to a technique adopted for spraying carbon nanotubes on a negative plate of a FED.
According to the present invention, the method provides a layer of a film sprayed by a solvent of a low viscosity and a high volatility with carbon nanotubes can expose the carbon nanotubes out of the layer of the solvent due to the spraying process under high-pressure air, because that particles from a binder or an additive with heavier specific weight deposit on a cathode or a surface of a negative glass substrate during the spraying process than that of the carbon nanotubes, so as to expose the carbon nanotubes out of the layer of the solvent under the high-pressure air for emitting large quantity of electrons and magnifying electric currents thereof.
2. Background of the Invention
The FED of the present invention is a device with a cathode electron emitter generating electrons within an electric field to excite phosphor materials covered on an anode thereof. The FED provides characteristics of lightweight and thin, sizes of an effective displaying area being adjustable to meet requirements, but without problems of view angles, which exist in a Flat LCD (Flat Liquid Crystal Display).
Referring to FIG. 1, a conventional FED 1a includes an anode 3a, a cathode 4a, and a rib 53 disposed between the anode 3a and the cathode 4a for separating the cathode 4 from the anode 3a, providing a support between the anode 3a and the cathode 4a, and including a vacuum cavity formed therebetween. See FIG. 1, the anode 3a includes a positive glass substrate 31a, an anode conductive layer 32a and a phosphor powder layer 33a; the cathode 4a includes a negative glass substrate 41a, a cathode conductive layer 42a and a electron emitting source layer 43a. With reference to the rib 53a in FIG. 1, the conventional FED 1a discloses functions, which the rib 53a provides, apparently for providing connections between the positive glass substrate 31a and the negative glass substrate 41a. By providing an electric field additionally, the electron emitting source layer 43a generates electrons and emits the electrons to the phosphor powder layer 33a of the anode 3a, and to avoid the positive glass substrate 31a electrically connecting the negative glass substrate 41a within the electric field, the rib 53a is supposed to be made of dielectric materials. According to a unit 5a shown in FIG. 1, includes an anode unit 51a and a cathode unit 52a, the rib 53a arranges between the anode unit 51a and the cathode unit 52a. Because the electrons generates due to the electric field (E), an electric field intensity thereof is in a direct proportion to a voltage adopted for the anode unit 51a and the cathode unit 52a, and in an inverse proportion to a distance between the anode unit 51a and the cathode unit 52a. The distance between the anode unit 51a and the cathode unit 52a obviously affect the electric field intensity thereof, therefore, even and uniform thickness of the electron emitting source layer 43a, the phosphor powder layer 33a and the rib 53a makes an important role in the conventional FED 1a and affect light uniformity thereof.
Recently, a newly carbon nanotube material is presented by Iijima in 1991 (see Nature 354, 56 (1991)). The newly carbon nanotube material provides a high aspect ratio, a high mechanical intensity and a high chemical resistance with a high abrasion resistance and a low threshold electric field, accordingly, the newly carbon nanotube material is applied to a filed emission electrons and to be studied broadly (referring to Science 269, p 1550 (1995); SID'98 Digest, p 1052 (1998); SID'01 Digest, p 316 (2001)). Wherein the so-called filed emission is a condition of a high electric field forcing on a material to diminish a thickness of an energy barrier thereof and electrons departing a surface of the material to be a free electron (with respect to J. Appl. Phys. 93, 7, pp 3504–3504 (1968)) by a quantum-mechanical tunneling effect. Thus, the electric currents due to the filed emission can be improved by the material surface with a low work function, the electrons generates without a constant heat source but with the material with the electric filed instead, and the filed emission device is called “cold cathode” thereby. The carbon nanotubes are adopted for the electron emitting source layer 43a of the negative glass substrate 41a of the conventional FED 1a. 
Methods for producing an electron emitting source layer with the carbon nanotubes are in different manners. In an CVD (Chemical Vapor Deposition) process, a carbon atom is deposited to continue each carbon nanotube of a negative substrate, although the method can provides a uniform length with stably growth to the carbon nanotubes, costs thereof are still high and sizes thereof are under 20 inches. To conquer the difficulty mentioned above, a method providing each layer thereof with a thick-film process results in the electron emitting source layer manufactured by screen printing and patents or articles in periodicals of the like; however, referring to FIG. 2, a electron emitting source layer 60a still has problems to solve. First, pastes for screen printing needs a high viscosity with at least 100,000 centi poise (cPs) to maintain precisions and configurations of design patterns thereof, besides the pastes further include binders (such as glass powders or the like), surfactants and conductive agents (such as silver or the like). And it's so difficult to disperse the carbon nanotube evenly with such a high viscosity and to avoid an effect of aggregation of the carbon nanotubes, which disturb the uniform distribution of the electrons, that the method needs dispersants in addition, the pastes have complex ingredients so as to increase the costs and to hazard dangerous oxides after sintering. Second, for processing the screen printing, thickness of each layer thereof is limited by a minimum thickness according to an emulsion of a fabric of a screen plate, with respect to FIG. 2, the thickness of each layer thereof 60a usually is at least 10 μm (micro meter), and a plurality of meshes arranged on the fabric of the screen plate differ 4˜8μm height therefrom to vary the thickness distribution of the design patterns. Third, the carbon nanotube 62a has a good aspect ratio, which even reaches 40, but in a normal case, the carbon nanotube 62a has to restrict a height thereof to scatter in the pastes to avoid influencing the printing process. Furthermore, the pastes embrace the carbon nanotube 62a thereof due to at least 10 μm thickness; part of the carbon nanotube 62a is still covered by the binders or conductive agents 61a even after sintering. Thus, efficiency of generating electrons decreases, for example, an electron emitting source layer with a multiple wall carbon nanotube is processed by the screen printing method, and when the electron emitting source layer is within a electric field (E) of 4 voltage per micrometer (V/μm), the electron emitting source layer still has a current density of 10 milliamperes per centimeter square (mA/cm2) limited.
Requirements we need to meet include: how to cut off the costs of the electron emitting source layer with the carbon nanotube; how to control or correct errors the thickness of the electron emitting source layer, so that the FED can provide a uniform display; how to expose the carbon nanotube out of the electron emitting source layer to increase the current density. Therefore, we can provide a spray and a method of spraying with a carbon nanotube on an electron emitting source layer to meet requirements mentioned above. First, simplifying ingredients of the spray to reduce the costs. Second, controlling a thickness of the electron emitting source layer by the spraying process. Third, exposing the carbon nanotube out of the electron emitting source layer by the spraying process to increase the current density.
Hence, an improvement over the prior art is required to overcome the disadvantages thereof.